Controlled Hydrothermal Synthesis and Photoluminescence of Nanocrystalline ZnGa2O4:Cr3+ Monospheres

The hydrothermal synthesis of nanocrystalline ZnGa2O4:Cr3+ (ZGC) red phosphor monospheres was accomplished in this work, and the effects of system pH, reactant content, reaction time, and citrate anions (Cit3-) on the phase and morphology evolution of the product were systematically studied. Under the optimized conditions of Cit3-/M = 1.0 molar ratio (M = total cations), pH = 5.0, and 0.2 mmol of Zn2+, well-dispersed ZGC monospheres with an average diameter of ~454 ± 56 nm (average crystallite size ~15 nm) were successfully obtained via hydrothermal reaction at 180 °C for 18 h. Cit3+ ions were demonstrated to be crucial to the formation of monospheres and substantially affect the pathway of phase formation. The ZGC monospheres calcined at 800 °C (average diameter ~353 ± 59 nm; average crystallite size ~30 nm) have an intensity ~6 times that of the original phosphor for the 700 nm red emission of Cr3+ (the 2E → 4A2 transition) under excitation with the O2- → Ga3+ charge transfer band at 250 nm. Fluorescence decay analysis found that the 700 nm emission has lifetime values of ~5 ms for the ZGC phosphors.